1. Field of the Invention
This invention relates to refractory materials for use in high temperature structural applications and a method for making these materials. More particularly, this invention relates to fibrous ceramic materials which can be formed into complex shapes in the green state and can be densified by pressureless sintering.
2. Discussion of the Prior Art
It is generally agreed that high fracture toughness is needed for ceramic materials used in high temperature structural applications in heat engines, as well as non-heat engine applications. Two aspects relating to the use of ceramics in high temperature structural applications are as follows:
(1) material performance; and
(2) shape-forming capability. High strength, fracture toughness and reliability (as measured by a Wiebul modulus) are desirable material properties for high performance applications. Ceramic fiber-ceramic matrix composites are currently of interest because of their desirable mechanical properties. Most of the work in this field, prior to 1983, is reviewed by D. C. Phillips, "Fibre Reinforced Ceramics," published in Handbook of Composites, Vol. 4, Fabrication of Composites, edited by A. Kelley and S. T. Mileiko, Elsevier Science Publishers, 373-428 (1983). Materials with good mechanical properties have been fabricated by hot-pressing chopped or continuous fibers mixed with a glass frit or metal oxide powder. The hot-pressing allows the glass frit or metal oxide powder to flow during densification thus producing a dense matrix surrounding the fibers. However, these mixtures cannot be pressurelessly sintered to high density because the fibers resist the shrinkage of the matrix powders leaving large voids between the fibers. The hot pressing process is limited in both the size and the shape of components which can be formed. The ability to form near-net-shapes is important for ceramic materials because machining is time-consuming and expensive. The machining process may also introduce strength limiting surface flaws.